In recent years, devices for processing digital audio signals have been actively developed. Accordingly, techniques related to digital audio signals for connecting devices have also been actively developed. With the movement toward downsizing of the devices such as portable telephones, consideration has been given to reducing the number of components.
There has been proposed a system in which 1 bit data array modulated by PDM method (Pulse Density Modulation) is transferred when audio data is communicated by an infrared data communicating device that conforms to the global standard for infrared data communication, i.e., IrDA (Infrared Data Association) (e.g. see Japanese Unexamined Patent Publication No. 135321/2004 (Tokukai 2004-135321, publication date: Apr. 30, 2004 (Patent Document 1)).
FIG. 10 is a block diagram illustrating a conventional infrared communication receiver 90. The infrared communication receiver 90 includes a receiving section 92. The receiving section 92 receives via infrared communication an audio signal, which is represented by 1 bit data array modulated by PDM method. Then, the receiving section 92 provides the data thus modulated to a pulse width expanding section 94. The pulse width expanding section 94 expands a pulse width of the audio signal received from the receiving section 92, and provides the audio signal to a speaker driving section 95. Based on the audio signal thus expanded by the pulse width expanding section 94, the speaker driving section 95 drives a speaker 96.
FIG. 11 is a circuit diagram illustrating a schematic of the pulse width expanding section 94, and FIG. 12 is a wave form chart representing its operation. The pulse width expanding section 94 includes a transistor Tr94, a resistor R94, a capacitor C94, and an inverter Inv94. A drain of the transistor Tr94, one end of the resistor R94, one end of the capacitor C94, and an input of the inverter Inv94 are connected to one another at a point A. The other end of the resistor R94 is connected to a terminal via which a power supply voltage Vdd is supplied. Further, the other end of the capacitor C94 and a source of the transistor Tr94 are connected to ground.
When an audio signal S94A having a pulse width of t_in is supplied to a gate of the transistor Tr94, an audio signal S94C is outputted from the inverter Inv94, based on an intersection of a signal S94B generated at the point A and a threshold voltage Vth of the inverter Inv94. The audio signal S94C has a pulse width t_out, which is longer than the pulse width t_in.
In this case, the pulse width is expanded by the following equation:Pulse width t_out=t_in+C×R×ln(Vdd/Vth),where t_in is a pulse width of the inputted audio signal S94A, and Vth is a threshold voltage Vth of the inverter Inv94.
As such, by expanding the pulse width of an audio signal in the pulse width expanding section 94 so as to increase a density of a pulse, a higher acoustic pressure can be obtained.
FIG. 13 is a circuit diagram illustrating a circuitry of a conventional monostable multivibrator 97, and FIG. 14 is a wave form chart representing its operation. The monostable multivibrator 97 includes an inverter Inv91, via which an audio signal is supplied. The output of the inverter Inv91 is connected to a capacitor C93. Opposite the inverter Inv91 relative to the capacitor C93, an inverter Inv92 and a resistor R93 are mutually connected in parallel. Further, at an opposed location to the capacitor C93, the resister R93 is connected to a terminal via which a power supply voltage Vdd is supplied.
On the output side of the inverter Inv92 are provided a transistor Tr97, a constant current I97, a capacitor C97, and a comparator Comp. A drain of the transistor Tr97, one end of a constant current I97, one end of a capacitor C97, and a non-inverting input terminal of the comparator Comp are connected to one another through a line C. The other end of the constant current I97 is connected to a terminal via which a power supply voltage Vdd is supplied, and the other end of the capacitor C97 and a source of the transistor Tr97 are connected to the ground. Further, an inverting input terminal of the comparator Comp is connected to a constant voltage source Vref1.
When an audio signal S97 in having a pulse width t_in is inputted to the inverter Inv91, a rising edge of an input pulse of the audio signal S97 in is detected, and a pulse signal S97A is outputted to the inverter Inv92 through a line A which connects the capacitor C93 and the inverter Inv92. Based on the pulse signal S97A, the inverter Inv92 generates a gate pulse S97B, and outputs it to a gate of the transistor Tr97. In response to the gate pulse S97B as a switch, a signal S97C having a waveform represented in FIG. 14 is inputted to the non-inverting input terminal of the comparator Comp through the line C. Then, the comparator Comp compares the signal S97C with the constant voltage source Vref1, so as to generate a signal S97out having a pulse width t_out. The pulse width t_out is found by the following equation:Pulse width t_out=tg+C97×Vref1/I97,where t_g is a pulse width of the gate pulse S97B, C97 is a capacitance, Vref1 is a constant voltage, and I97 is a constant current.
The monostable multivibrator 97 shown in FIG. 13 stabilizes a pulse width of an audio signal received in an IrDA device.
Further, a receiving circuit is disclosed which has been known as avoiding generation of unwanted pulses caused by a variation in voltage of an outputted signal (see Japanese Unexamined Patent Publication No. 130088/2005 (Tokukai 2005-130088, publication date: May 19, 2005 (Patent Document 2)).
However, the monostable multivibrator 97 shown in FIGS. 13 and 14 has a problem in that a pulse width is constant and cannot be expanded or compressed for adjustment, although it is capable of stabilizing a pulse width.
Further, in the infrared communication receiver 90 described with reference to FIGS. 10 through 12, a pulse width is merely expanded to be larger than an input pulse width and cannot be compressed. Thus, the infrared communication receiver 90 cannot reduce an acoustic pressure of a speaker by compressing a pulse width. Further, the infrared communication receiver 90 requires the pulse width expanding section 94 as well as the receiving section 92 (IrDA device). This causes an increase in the number of components, further causing difficulty in downsizing.